SICT. XXX. THE MAGNETIC ATLAS. 313 



terval of longitude than in the north hemisphere. Their 

 respective longitudes, derived from the observations of 

 the antarctic expedition which Colonel Sabine has re- 

 duced and published in the Phil. Trans., are approxi- 

 mately 130 and 220 east. The latitudes are not de- 

 rivable from the observations with equal approximation ; 

 but they do not appear to differ much from the corres- 

 ponding latitudes in the north ; i. e. the stronger about 

 50 or 55 south, and the weaker about 70 south. Here 

 also the weaker maximum has a very considerable sec- 

 ular movement, amounting, as Colonel Sabine has given 

 reason to believe in the Phil. Trans, of last year, to 

 nearly 50 of longitude in 250 years : the secular change 

 in the southern hemisphere being to the westward, 

 while that in the northern is to the eastward. 



The dip of 90 is far removed from either of these 

 localities ; its approximate position may be called about 

 73 south and 147 east; but the isoclinal curve of 89 

 will doubtless be more correctly given when the Pagoda 

 returns from the completion of the survey, and when 

 the whole of the observations in the southern hemis- 

 phere are combined and treated according to the formulae 

 of the * Allgemeine Theorie." 



The object of the geographical branch of the magnetic 

 observations of the last few years has been to obtain 

 determinations, with the improved instruments of the 

 present time, in every accessible part of the globe, with 

 a view of combining the results into magnetic charts of 

 the three elements drawn directly from the observations, 

 and corresponding to the present epoch. The Magnetic 

 Atlas will then be recomputed by the methods described 

 in Gauss' " Allgemeine Theorie." The observation part 

 is nearly accomplished. 



(a) This is by no means established ; the distribution 

 of land and water appears to have considerable influence 

 on the form of the magnetic equator, as Mrs. Somer- 

 ville states at (6). 



(c) In the balance of torsion, the intensity of electrical 

 forces is not measured by oscillations, but by the torsiojj 

 necessary to destroy the deviation produced* 



(d) Refer to note (4). 



Dn 



